This invention generally relates to a reversible switched reluctance motor operating without shaft position sensors and, particularly, to a circuit for varying a speed and direction parameter for controlling speed and direction of rotation in such a switched reluctance motor.
Generally, switched reluctance motors, also called variable or electronically commutated reluctance motors, are doubly salient having poles or teeth on both a stator and a rotor. Such motors include windings on the stator poles but not on the rotor poles. Typically, each pair of diametrically opposite stator windings is connected in series to form one phase of the switched reluctance motor.
The switched reluctance motor produces torque by commutating the phases in a predetermined sequence so that a magnetic force of attraction results between rotor and stator poles that are approaching each other. A converter switches off the current in each phase at a commutation point before the rotor poles nearest the stator poles corresponding to that phase rotate past an aligned position. The switched reluctance motor uses a single unidirectional current switching element, such as a thyristor or transistor, in each leg of the converter to generate unidirectional current pulses synchronized with rotor movement and to supply the current to the corresponding phase of the motor. Thus, the motor develops torque which is independent of current direction.
Each time the converter switches on a phase of the switched reluctance motor, current from a DC supply flows in a pair of stator windings. The motor converts energy drawn from the supply partly into a magnetic field and partly into mechanical energy as the rotor rotates toward a minimum reluctance configuration. When the conducting switch is opened, the motor converts part of the stored magnetic energy into mechanical output and preferably returns the remainder of the energy to the DC source.
In operation, the converter must switch the phase current on and off in precise synchronism with the position of the rotor. Typically, a shaft position sensor is used to reference the switching of the transistors in each converter leg to a set of pulses derived from the shaft position sensor to accomplish this "shaft-position switching". It is known to operate a variable reluctance stepping motor without a shaft position sensor and without loss of steps by a method wherein the converter provides very wide current pulses without reference to the rotor position. The stepping motor thus operates at a very high torque margin so that torque transients do not cause a loss of steps. While the method described for operating stepping motors has high stability, the method obtains the high torque margin at the expense of efficiency. It is also known to operate a variable reluctance motor without a shaft position sensor at high efficiency by a method wherein changing a dwell or conduction angle of the phase currents in response to a change in load torque allows dynamic increase of the torque margin. While this method provides high efficiency, it does not provide a scheme for efficiently reversing the direction of rotation in the motor.
Variable reluctance motors, including switched reluctance motors, are disclosed in U.S. Pat. Nos. 4,611,157, 4,707,650, 5,012,171 and 5,012,172, all of which are commonly assigned with the present application and the entire disclosures of which are incorporated herein by reference in their entirety.